Optical devices utilizing optoelectronic chips such as lasers and photodetectors (PD) require coupling the light to and from optical fibers. Such devices can be found for example in telecom or Datacom applications, fiber sensors and various medical diagnostic applications. The complexity of fiber coupling increases significantly as the fiber core diameter decreases since the spot size of the optoelectronic chip needs to be similar to that of the fiber. In addition, the numerical aperture (NA) on both sides should match to allow efficient coupling within the fiber angular acceptance cone. Fibers used in optical communication applications are either multimode or single mode with a core diameter of 50 and 9 micron, respectively.
The small core diameter imposes stringent limitations on the coupling efficiency that are directly affecting the overall cost of the device. In principle, the light from a laser source is routed via a lens to the fiber input face. On the opposite side of the optical link, light coming out from the fiber is focused with a lens such that all of the light is incident on the PD aperture. As Telecom and Datacom data rate increase beyond 25 Gb/s to 50 Gb/s and higher, the apertures of both lasers and PD decrease making fiber coupling more complicated since both the spot size and angular distribution are more difficult to control.
High efficiency fiber coupling can be achieved if an active alignment scheme is used. This scheme involves powering up all electrical and optoelectronic chips on the device and carrying out the alignment while monitoring the laser optical power coupled into the fiber, or by monitoring the photocurrent resulting from coupling light from the fiber to the PD. It can be appreciated that active fiber alignment is a costly and labor-intensive process that is not suitable to volume production of optical modules.